This invention relates to a variable capacitance tuning circuit for high-frequency signals which is used in a television receiver, has excellent interference-eliminating ability and noise characteristics, and can tune to multiple channels.
Description will be made of a tuner for receiving multiple channels of television including cable television.
In North America or European countries such as Belgium, multi-channel cable television has been developed and now the number of broadcast channels is increasing. Thus, since the number of VHF channels to be received by a television tuner is increasing it is now necessary to use a high-performance television tuner capable of receiving more channels.
A television tuner capable of tuning to multiple channels is constructed as shown in FIG. 1. In this figure, A represents an antenna tuning circuit for receiving predetermined frequencies of antenna signals, B an RF amplifier, C an interstage double tuning circuit and D a mixer circuit.
In addition, 1 represents a VHF antenna input terminal, 2 a band-I tuning coil in the antenna circuit, 3 a band-II tuning coil therein, 4 a band-III tuning coil therein, 5 a varactor diode, 6 another varactor diode, 7 a switch circuit, 8 an amplifying MOS-FET, 9 an interstage primary band-I tuning coil, 10 an interstage secondary band-I tuning coil, 11 an interstage primary band-II tuning coil, 12 an interstage secondary band-II tuning coil, 13 an interstage primary band-III tuning coil, 14 an interstage secondary band-III tuning coil, 15 a mixing MOS-FET and 16 a tuning circuit.
FIG. 2 shows the tuning circuit extracted from FIG. 1. A receiving end or feeding end 17 is connected to the junction between the varactor diodes 5 and 6. This junction is connected to a BT terminal, that is, a terminal to which a D.C. tuning voltage is applied to change the capacitance value of the varactor diodes 5 and 6 thereby changing the resonant frequency of the tuning circuit.
This tuning circuit, however, has too small a variable capacitance to tune to more channels. Thus, it is necessary to develop a television tuning circuit capable of tuning to multiple channels.
Also, as compared with the tuner for receiving 12 VHF channels or so, it is difficult with this tuning circuit to add an image frequency trap for reception of multiple channels and thus it has a poor image interference ratio. It is also desired to use a tuning circuit which has an excellent noise figure.